The flow of oil from a subterranean formation to a well bore depends on various factors, including permeability of the formation. Often, permeability of the formation is insufficient to allow a desired flow rate of fluids, such as oil and gas, from the formation. In these cases, the formation can be treated to increase permeability.
Acidizing limestone and dolomite formations with HCl acid is one method to increase the yield of oil and gas from the formation. The limestone or dolomite formation can be stimulated by pumping HCl acids down the wellbore tubing, casing, or thru coiled tubing. The HCl acid is then injected into the formation to dissolve the limestone or dolomite rock, thereby forming a conductive channel extending from the wellbore into the formation area containing the oil and gas. At the conclusion of the acid treatment the spent acid can be recovered from the formation at the surface.
The most common acid utilized to stimulate limestone or dolomite formations is HCl, in strengths ranging from about 7.5% to about 28% by weight. The most common acid strength utilized for acid stimulation is 15% HCl. HCl acid treatments are usually formulated with fresh water, 32% HCl acid stock, and other additives such as corrosion inhibitor, iron control agents, and clay stabilizers.
The acid system formulated with fresh water reacts with the limestone or dolomite formation to form by-products of calcium chloride liquid brine and carbon dioxide gas. Acid systems mixed with fresh water form little or no solid precipitates, allowing the acid to freely react with the limestone or dolomite rock to form a straight conductive channel into the formation. The goal of the acid stimulation is to form a long extended conductive channel deep into the productive zone.
Acid stimulation fluids are usually mixed with fresh water, but if fresh water supplies are limited or unavailable, seawater is sometimes substituted for fresh water in part or full. One specific area where this often occurs is the offshore boat stimulation market.
While an acid treatment formulated with fresh water results in little or no solid precipitation formation, major solid precipitation problems in the formation can arise when acids are mixed with seawater. Stimulation of limestone or dolomite formations with HCl acid systems formulated with seawater achieves less than desirable stimulation results because of this solids precipitate. When acid systems are formulated with seawater containing high levels of sulfate ions, calcium sulfate precipitates as the acid reacts with limestone or dolomite formation. The extent of calcium sulfate deposition or scaling, although sometimes accepted or ignored by some customers, can result in post stimulation results far below their true potential.
Historically, various technologies can remove sulfite from seawater, thereby reducing precipitate formation. However, these technologies are expensive and logistically challenging or impractical to use on offshore installations or conventional stimulation vessels. These various technologies include ion exchange, ion specific resins, or barium chloride precipitation.
A cost effective method to chemically reduce calcium sulfate precipitation from sulfate-laden water during acid reaction of the stimulation process would be very advantageous. Such a chemical treatment system could provide one or more benefits, such as reducing the need to pre-treat seawater, simplifying logistics or increasing stimulation efficiency.